(1) Field of the Invention
The present invention relates to an optical transmission system wherein two terminal stations are connected with at least one working system optical transmission line and one protection system optical transmission line, each terminal station contains pieces of optical working system terminal equipment and the protection system corresponding to the optical transmission lines which the terminal station accommodates, and the optical protection system terminal equipment and the optical working system terminal equipment are connected with an optical transmission line in the terminal station. Explanations are given in the above application Ser. No. 07/784,211 for various aspects of such an optical transmission system.
In particular, the characteristic feature of the present invention resides in a control operation in the optical protection system terminal equipment when trouble occurs in the protection system.
(2) Description of the Related Art
FIG. 1 is a diagram illustrating an example construction of an optical transmission system wherein two terminal stations are connected with at least one working system optical transmission line and one protection system optical transmission line, each terminal station contains pieces of optical working system terminal equipment and the protection system corresponding to the optical transmission lines which the terminal station accommodates, and the optical protection system terminal equipment and the optical working system terminal equipment are connected with an optical transmission line in the terminal station.
In FIG. 1, reference numerals 1' and 2' each denote a piece of protection system terminal equipment; 3', 4', 5', and 6' each denote a piece of working system terminal equipment, PTCT1 and PTCT2 denote a pair of protection system optical transmission lines where one of the pair transmits optical signals in one direction and the other of the pair transmits optical signals in the opposite direction; WK11 and WK12 denote a first pair of working system optical transmission lines where one of the pair transmits optical signals in one direction and the other of the pair transmits optical signals in the opposite direction; WK21 and WK22 denote a second pair of working system optical transmission lines where one of the pair transmits optical signals in one direction and the other of the pair transmits optical signals in the opposite direction; REG01 to REG22 each denote an optical regenerating repeater; 7, 7', 8, and 8' each denotes an optical transmission line in each terminal station; 91, 92, 95, and 96 each denotes an optical coupler; and 93, 94, 97, and 98 each denotes an optical switch. Further, in each terminal equipment, 0S1 and 0S2 denote a first and second optical transmission portions, and 0R1 and 0R2 denote a first and second optical reception portions.
In FIG. 1, the protection system terminal equipment 1', the pieces 3' and 5' of working system terminal equipment, the optical transmission lines 7 and 7', the optical couplers 91 and 92, the optical switches 93 and 94, and a switch control circuit (not shown), are provided in one of the terminal stations; and the protection system terminal equipment 2', the pieces 4' and 6' of working system terminal equipment, the optical transmission lines 8 and 8', the optical couplers 95 and 96, the optical switches 97 and 98, and a switch control circuit, are provided in the other of the terminal stations.
In each working system terminal equipment, tributary (electric) signals are multiplexed by the multiplexer (denoted by MUX in the drawing), and the multiplexed electric signals are supplied to the first and second optical transmission portions 0S1 and 0S2 in parallel. The multiplexed electric signals are converted to an optical signal in the first and second optical transmission portions 0S1 and 0S2 to output the same onto the optical transmission line.
In the above construction, the first optical transmission portion 0S1 in each piece of working system terminal equipment accommodates one of the working system optical transmission lines WK11 to WK22, which is connected to the opposite terminal station (the other terminal station which is located opposite to the terminal station containing the above piece of working system terminal equipment). An optical signal output from the first optical transmission portion 0S1, is transmitted through the working system optical transmission line, regenerated by one of the optical regenerating repeaters REG01 to REG22 which are provided on the optical transmission line. Then, the optical signal is received by the first reception portion 0R1 in the corresponding piece of working system terminal equipment in the opposite terminal station. The received optical signal is converted to an electric signal in the first reception portion 0R1. When the working system optical transmission line is used, the demultiplexer, which is denoted by DMUX in the drawing, is connected on the output side of the first reception portion 0R1 in each working system terminal equipment. The above (multiplexed) electric signal output from the first reception portion 0R1, is demultiplexed in the demultiplexer, and is then output as a tributary (electric) signal onto a transmission line on the tributary side.
In the above construction, when trouble occurs, for example, in the optical transmission line WK11, as indicated by "A" in FIG. 1, the optical regenerating repeater REG11, which is located downstream of the location of the trouble, detects the trouble as an abnormality in a signal received thereat. For example, in the SONET (Synchronous Optical Network) system, the optical regenerating repeater REG11 generates an alarm indication signal (LINE AIS) in response to the abnormal signal to inform the terminal station located downstream of the optical regenerating repeater REG11 about the abnormality. When a reception portion 0R1 detects loss of a receiving signal, or the above alarm indication signal (LINE AIS), the reception portion 0R1 sends a switch trigger signal (SW TRG) to the switch control circuit in the same terminal station, and activates the first transmission portion 0S1 in the same working system terminal equipment to transmit a far end receive failure signal (FERF) to the working system terminal equipment 3' through the other optical transmission line WK12 which makes the pair together with the above optical transmission line through which the above reception portion 0R1 received the alarm indication signal (LINE AIS). When the first reception portion 0R1 in the working system terminal equipment 3' receives the above far end receive failure signal (FERF), the first reception portion 0R1 sends a switch trigger signal (SW TRG) to the switch control circuit in the same terminal station.
The switch control circuit in each terminal station monitors an output of the switch trigger signal (SW TRG) from the reception portion in each working system terminal equipment and protection system terminal equipment in the terminal station, and determines which working system terminal equipment is to be switched to the protection system optical transmission line. Based on the above determination, the switch control circuit connects each working system terminal equipment with a working system optical transmission line, or with the protection system terminal equipment through the optical transmission lines 7 and 7', or 8 and 8' in each terminal station. For example, in the construction of FIG. 1, when one of the pieces of working system terminal equipment is connected to the corresponding one of the working system optical transmission lines, one of the optical switches (a corresponding one of the switches 93, 94, 97, or 98), which receives an output of the second transmission portion 0S2 in the working system terminal equipment, stops the output, and the switch control circuit makes a connection so that the demultiplexer DMUX in the working system terminal equipment selects the output of the first reception portion to input thereto. On the other hand, when each working system terminal equipment is connected to the protection system terminal equipment through the optical transmission lines 7 and 7', or 8 and 8' in the terminal station, one of the optical switches (a corresponding one of the switches 93, 94, 97, or 98), which receives an output of the second transmission portion 0S2 in the working system terminal equipment, selects the output to input thereto, and the switch control circuit makes a connection so that the demultiplexer DMUX in the working system terminal equipment selects the output of the second reception portion to input thereto. Thus, the second reception portion is connected through the optical coupler (a corresponding one of the optical couplers 91, 92, 95, and 96) and optical transmission line 7' to the protection system terminal equipment in the same terminal station.
Further, as explained in the copending application Ser. No. 784, 211, when switches are provided so that the output of the multiplexer MUX in each working system terminal equipment is supplied to only one of the first and second transmission portions 0S1 and 0S2, each of the optical switches 93, 94, 97, and 98 may be replaced with an optical coupler. When the optical switches 93, 94, 97, and 98 are replaced with optical couplers, the switch control circuit in each terminal station controls the second transmission portion in each working system terminal equipment so that only one second transmission portion in one piece of working system terminal equipment, which is to be connected to the protection system terminal equipment, outputs an optical signal to the corresponding optical coupler.
Each protection system terminal equipment receives at the second reception portion 0R2 thereof the output of the second transmission portion 0S2 of the above one working system terminal equipment in the same terminal station through the optical transmission lines 7 or 8' in the terminal station, converts the output to an electric signal, and then converts the electric signal to an optical signal in the first transmission portion 0S1 thereof to transmit the same through the protection system optical transmission line PTCT1 or PTCT2 to the protection system terminal equipment in the opposite terminal station (which is referred to below as a second terminal station).
The protection system terminal equipment in the second terminal station receives at the first reception portion 0R1 thereof the above optical signal transmitted through the protection system optical transmission line PTCT1 or PTCT2, converts the optical signal to an electric signal, and then converts the electric signal to an optical signal in the second transmission portion 0S2 thereof to transmit the same through the optical transmission line 7' or 8 in the second terminal station to one of the pieces of the working system terminal equipment in the second terminal station. This piece of the working system terminal equipment is located opposite to the above piece of working system terminal equipment, connected to the protection system terminal equipment in the above terminal station which is opposed to the second terminal station. As explained above, in the construction described in the copending application Ser. No. 784,211, each protection system terminal equipment in each terminal station has a function of an optical regenerating repeater provided between an protection system optical transmission line and an optical transmission line in the terminal station.
A case is considered below wherein trouble "B" occurs in the protection system optical transmission line PTCT1, as indicated in FIG. 2, in the condition that the switching to the protection system optical transmission line is made.
In this case, the trouble is detected as an abnormality in a received signal at the optical regenerating repeater REG01 downstream of the location of the trouble. In response to the abnormal signal, the optical regenerating repeater REG01 generates an alarm indication signal (LINE AIS) to inform the protection system terminal equipment 2', downstream of the optical regenerating repeater REG01 of the abnormality. When each reception portion 0R1 in the protection system terminal equipment 2' detects a loss of a receiving signal or the above alarm indication signal (LINE AIS), the reception portion 0R1 sends a switch trigger signal (SW TRG) to the switch control circuit in the same terminal station, and activates the first transmission portion 0S1 in the same working system terminal equipment to transmit a far end receive failure signal (FERF) to the protection system terminal equipment 1' through the other optical transmission line PTCT2 which makes the pair together with the above optical transmission line through which the above reception portion 0R1 received the alarm indication signal (LINE AIS). When the first reception portion 0R1 in the protection system terminal equipment 1' receives the above far end receive failure signal (FERF), the first reception portion 0R1 sends a switch trigger signal (SW TRG) to the switch control circuit in the same terminal station.
According to the function of the optical regenerating repeater, each of the pieces 1' and 2' of protection system terminal equipment supplies the alarm indication signal (LINE AIS) or far end receive failure signal (FERF) which is received at the first reception portion 0R1 thereof, to the pieces of working system terminal equipment in the same terminal station, which pieces are connected to the above each piece 1' and 2' of protection system terminal equipment through the optical transmission lines 7' and 8 in each terminal station. Each of the first and second reception portions in each working system terminal equipment has a function of generating a switch trigger signal (SW TRG) to transmit the same to the switch control circuit in the same terminal station when the alarm indication signal (LINE AIS), the far end receive failure signal (FERF), or, the loss of an input is received. Therefore, the switch trigger signals (SW TRG) are simultaneously generated from the second reception portions 0R2 in all the working system terminal equipment in each terminal station, to be transmitted to the switch control circuit in each terminal station, since the alarm indication signal (LINE AIS) or far end receive failure signal (FERF) is simultaneously supplied to the second reception portion 0R2 in all the pieces of working system terminal equipment from the protection system terminal equipment, which functions as an optical regenerating repeater. In this case, the switch control circuit in each terminal station cannot recognize the real situation as to which optical transmission line trouble has occurred.
In addition, when trouble occurs on the optical transmission line 8 in the terminal station as indicated by "C" in FIG. 2, the trouble is simultaneously detected as loss of a signal input in the second reception portions 0R2 in the pieces of working system terminal equipment, which pieces are located downstream of the location "C", and the above switch trigger signal (SW TRG) is transmitted to the switch control circuit. Therefore, when the above trouble "B" on the protection system transmission line PTCT1, and the trouble "C" on the optical transmission line 8 in the terminal station simultaneously occur, the switch control circuit cannot recognize the real situation.
Further, switches are provided so that the output of the multiplexer MUX in each working system terminal equipment is supplied to only one of the first and second transmission portions 0S1 and 0S2, and the optical switches 93, 94, 97, and 98 are respectively replaced with optical couplers, while far end receive failure signals (FERF) are output from the second transmission portions 0S2 in all the pieces of working system terminal equipment, in response to the alarm indication signal (LINE AIS), where the alarm indication signal (LINE AIS) is supplied to all the pieces of working system terminal equipment. The far end receive failure signals (FERF) are input to the second reception portion 0R2 in the protection system terminal equipment through the respectively corresponding optical couplers 93, 94, 97, and 98, where the far end receive failure signals (FERF) are superimposed with each other at the input. Therefore, there is a risk that the second reception portion 0R2 in the protection system terminal equipment is damaged due to receipt of an optical signal having an intensity more than a predetermined intensity.
It may be possible to preset the terminal stations so that only one of the plurality of pieces of working system terminal equipment can output the far end receive failure signal (FERF) to prevent the input of the superimposed signal at the reception portion 0R2. However, in this case, the optical/electric converters 0R2 in the pieces 3' and 5' of working system terminal equipment will receive the far end receive failure signal (FERF) which is commonly transmitted thereto. Therefore, it is difficult to determine which piece of terminal equipment is to be switched to the protection system.